The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Color enhancement is a common processing task used in video equipment and displays. The color enhancement is usually performed on digital video data using logic implemented by dedicated hardware on an application-specific integrated circuit (ASIC). Such an implementation necessitates that the enhancement function be easy to implement and not cause any side effects in terms of artifacts that would be visible in the enhanced image. Typical improper color enhancement artifacts include banding or clipping artifacts or loss of detail in the displayed image.
Digital video is widely available in YCbCr or YUV color spaces and thus it is common to perform the color enhancement in these color spaces. Conventional color enhancement processes include changing the brightness, contrast, saturation and hue of the pixels in an image. Adaptive color enhancement schemes selectively enhance certain colors in an image, e.g. blue in the sky and green in the grass, while at the same time preserving other colors, e.g., skin colors.
In typical adaptive color enhancement schemes, valid color values are calculated for each pixel, i.e. the resulting red, green and blue values of the pixel are within valid range, e.g. between 0 and 1 in the normalized range of values. However, this approach does not ensure that the resulting color is displayable on any particular digital display, since the digital display has its own set of color constraints. The color constraints of a digital display are often specified in terms of the color gamut of the display.
A color gamut refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device. If the display color space gamut (or any other target color space gamut) is not taken into account during color enhancement, it is possible that a pixel would be in valid range within a particular color space from a numerical viewpoint, but the pixel may not be displayable in the color gamut. A clipping artifact can occur when a pixel that is not displayable in a color gamut is clipped to the boundary of the color gamut. Similarly, a banding can occur when two colors that were close in the unenhanced state become spaced farther apart due to the color enhancement. Another artifact of color enhancement is a coloration artifact in which achromatic colors show coloration after color enhancement. Even when typical color enhancement processes attempt to reduce the occurrence of these and other color enhancement artifacts, they do not take into account potential variations between color gamuts and therefore cannot prevent any of the artifacts that occur when some of the color enhanced pixels cannot be accurately represented within a particular color gamut.
In addition, typical color enhancement techniques do not take into account the differences between the color gamut of a digital video content source and the color gamut of a digital video content display. Digital video content is often constrained to a small color gamut due to the acquisition systems involved in the formation of such content. The compression schemes that are used for the storage and delivery of such digital video content also often use a small color gamut. Most of the current video content is rendered for conventional CRT displays. Conventional CRT displays use a standard sRGB color gamut. More modern display devices such as LCD displays have emerged with a much larger color gamut relative to sRGB. Viewers do not get the benefit of the extended gamut displays if the sRGB content is rendered on the extended gamut displays.